1. Field of the Invention
The present invention relates to a gear ratio selecting apparatus in an automatic transmission (T/M). This apparatus has a synchromesh mechanism.
2. Description of Related Art
PCT Application Unexamined Publication WO 97/05410 discloses a gear ratio selecting apparatus in a transmission. This apparatus controls working force of a hydraulic actuator in response to synchromesh mechanisms, shifting steps, operating modes such as a sports mode and an economy mode, or operational states of the synchromesh mechanisms during shifting operation.
This apparatus disclosed in the publication mainly includes: a shifting actuator for moving a shifting fork; a selecting actuator for moving a selecting lever; a proportional flow regulating valve for controlling a hydraulic fluid flow to be supplied to the shifting actuator and the selecting actuator during shift operation; a changing valve for supplying a hydraulic fluid discharged from a hydraulic pressure source to the proportional flow regulating valves; and a pressure control valve for controlling a hydraulic pressure in response to a position of the selecting lever.
The publication also discloses a technique in which the changing valve has a pressure-controlling function to omit the abovementioned pressure control valve so as to control the working force during the shift operation.
As shown in FIG. 6, the shifting actuator disclosed in the publication has a double acting cylinder equipped with a rod extending in one direction, the hydraulic pressure source supplies a primary pressure "PO" to a hydraulic chamber 1 and it supplies a controlled pressure "P1" to the other hydraulic chamber 2. Since the pressure of the hydraulic chamber 1 is constant, a piston 3 is moved by changing the pressure of the hydraulic chamber 2 in one direction and the other direction. This shifting actuator shown in FIG. 6 has a problem that the piston 3 does not accurately return to a neutral position thereof because of dispersion of the controlled pressure "P1".
In the above-mentioned shifting actuator shown in FIG. 6, a pressure-receiving area "S1" is different from a pressure-receiving area "S2" in the hydraulic chamber 1. Accordingly, even when the apparatus supplies the same hydraulic pressure to both of the hydraulic chambers 1,2, the piston 3 does not return to the neutral position.
Also, the above-mentioned shifting actuator can not obtain a large propulsive force for the piston 3 to move to the left side, since the propulsive force "FL" is smaller than the propulsive force "FR" (FR&gt;FL).
Further, the above-mentioned shifting actuator has a problem in durability, since a high hydraulic pressure, the primary hydraulic pressure "PO", is usually supplied in the hydraulic chamber 1. Increasing durability requires improvement in parts precision, material changes and the like, thereby increasing costs disadvantageously.